1. Field of the Invention
The present invention relates generally to a polishing apparatus. More specifically, the invention relates to a polishing apparatus for planarizing roughness or unevenness on a surface of a semiconductor substrate or the like.
2. Description of the Related Art
In the recent years, in a fabrication process of a semiconductor, a polishing process has been used for planarization of roughness or unevenness of a semiconductor substrate or of step of element and wiring on the surface of the semiconductor substrate. The polishing process typically employs a system to selectively polish protruding portion for removal and whereby to achieve complete planarization.
Constructions of the conventional polishing apparatus typically employed have been disclosed in Japanese Unexamined Patent Publication No. Showa 59(1984)-187456, Japanese Unexamined Patent Publication No. Heisei 7(1995)-221053 and Japanese Unexamined Patent Publication No. Showa 58(1983)-22657. For example, as shown in FIG. 4(a), the typical polishing apparatus is constructed with a polishing object holding means (hereinafter referred to as "fixing block") 302 for holding a polishing object 101 via a buffering plate 303, such as a semiconductor wafer or the like and rotating carrying the polishing object, and a rotary polishing table 106. The buffering plate 303 is necessary for absorbing a load locally exerted on the polishing object 101 and whereby for improving uniformity of a polished surface. As the buffering plate, DF200 manufactured by Rodel, U.S.A. has been generally used. DF200 has a laminated structure of a polyurethane foamed layer and polyester sheet base material and has a compressibility factor (in accordance with a standard of JIS L-1096) of 33%. For instance, DF200 adapted for 6 inch wafer has an external diameter of 150 mm and a thickness of 0.6 mm. On the other hand, in order to prevent the polishing object 101 from loosing off the fixing block 302, a retainer ring 304 having an internal diameter of 151 mm and a thickness of 7 mm is engaged on the outer periphery of the fixing block 304.
On an upper surface of the rotary polishing table 106 having a rotary drive shaft 105A, an abrasive cloth is bonded. The abrasive cloth is typically used with lapping over an abrasive cloth 107 (upper layer) and an abrasive cloth 108 (lower layer). As the abrasive cloth 107, IC1000 manufactured by Rodel, is frequently used. The abrasive cloth 107 is formed by foaming and hardening urethane resin and is hard having a hardness factor of 95 in an Asker C hardness meter. On the other hand, as the abrasive cloth 108, Suba400 manufactured by Rodel Nitta Company is frequently used, which is formed by impregnating polyurethane resin into a polyester fiber non-woven fabric and soft having a hardness factor of 61 in the Asker C hardness meter. Functions of the abrasive cloths 107 and 108 is that the abrasive cloth 107 serves for increasing flatness and the abrasive cloth 108 serves for absorbing camber or undulation of the surface of the polishing object 101 to improve uniformity of the polishing surface. On the other hand, a polishing fluid supply nozzle 109 is provided at the center portion above the abrasive cloth 107.
Next, operation of the foregoing polishing apparatus under typical polishing condition will be discussed. When a polishing fluid 110 containing solid component, such as silicon oxide or the like is supplied in a rate of 200 cc/min on the abrasive cloth 107 rotating at a rotation speed of 30 r.p.m., the polishing fluid 110 propagates along the upper surface of the abrasive cloth 107 from the center portion to the end portion. Under this condition, the polishing object 101 held on the fixing block 302 is fitted on the abrasive cloth 107 under pressure. Then, polishing is performed with applying a 7 psi (not shown) of load on the fixing block 302. At this time, in order to certainly maintain uniformity of in-plane polishing speed of the polishing object 101, the fixing block 302 is also rotated on own axis at a speed of 30 r.p.m.
FIG. 4C is a chart showing a polishing speed in a direction including a diameter extending cross the center of the polishing object 101 polished by an apparatus shown in FIG. 4A and extending perpendicular to a polishing surface. The shown profile is the same in the profile of any diametrical direction taking the center of the polishing object 101 as an origin. Since FIG. 4C is a bilateral symmetric profile with respect to the origin, discussion will be given for the polishing speed of the polishing object 101 from an edge in the plus direction to the origin.
At the edge portion (a region of +73 to +75 mm) of the polishing object 101, the polishing speed is quite high. Conversely, in a region of +60 to +73 mm, the polishing speed is low, and has a minimum point in a region of +71 to +73 mm. In a region of 0 to +60 mm, the polishing speed is uniform. A tendency of the polishing speed at the end portion of the polishing object 101 is caused due to difference of pressure exerted on the polishing object 101 from the abrasive cloth 107. Since the abrasive cloth 107 in the upper layer is hard but the abrasive cloth 108 in the lower layer is soft, the abrasive cloths 107 and 108 mating with the edge portion of the polishing object 101 is locally deformed. FIG. 5B shows a result of measurement of a pressure exerted on the edge portion of the polishing object 101 by means of a pressure sensor. In the region of +73 to +75 mm, due to large deformation of the abrasive cloths 107 and 108, an excessively high pressure is exerted locally. Conversely, in the region of +60 to +73 mm, the pressure is gradually released and lowered. Then, in the region of 0 to +60 mm, the pressure is uniformly exerted. The pressure distribution in respective of these regions correspond to distribution of the polishing speed.
In order to improve non-uniformity of the polishing speed, there is a method to use Suba800 manufactured by Rodel Nitta Company, which is formed of the same material as Suba400 but having higher hardness. Suba800 has a hardness factor of 82 in Asker C hardness meter. FIG. 6A is an enlarged section of the edge portion in the case where Suba800 is used as the abrasive cloth 108 in the apparatus of FIG. 4A. While it is similar even in the case using SuBa400, since the buffer plate 303 is used, displacement is caused between the buffering plate bonding surface of the fixing block 302 and the buffering plate 303 is caused to form a gap portion 316 between the end portion of the buffering plate 303 and the retainer ring 304. The enlarged section shown in FIG. 6A shows an example causing plus side displacement of the buffering plate 303, in which the plus side edge of the polishing object 101 is located completely above the buffering plate 303.
Discussion for the pressure distribution and the polishing speed at the plus side edge portion will be given with reference to FIG. 6B. In this case, Suba800 as the abrasive cloth 108 of the lower layer is hard, local deformation amount of the abrasive cloths 107 and 108 at the edge portion of the polishing object 101 is small even when the polishing object 101 is depressed onto the abrasive cloths 107 and 108. Therefore, in the region of +73 to +75 mm of FIG. 6B, excessive pressure may be exerted by shifting of the polishing object 101 during polishing. Thus, the pressure exerted from the abrasive cloth 107 is equivalent to the case where Suba400 is used. In the region of +60 to +73 mm, since local deformation amount is small, the pressure to be exerted on the polishing object 101 becomes uniform. Accordingly, as shown in FIG. 7, in a region of +60 to 73 mm, the polishing speed becomes uniform to make the uniform region wider than that in the case where Suba400 is used.
It should be noted that displacement between the buffering plate bonding surface of the fixing block 302 and the buffering plate 303 is caused by the structure of the fixing block 302. Therefore, assembling method of the buffering plate will be discussed with reference to FIGS. 4A and 4B. For example, in the fixing block 302 for 6 inch wafer, on the buffering plate bonding surface having an internal diameter of 151 mm, the buffering plate 303 having an external diameter 150 mm which is smaller than the bonding surface in the extent of 1 mm, is bonded. Next, the retainer ring 304 is engaged to the fixing block 302 and fixed by means of screws. The reason while the buffering plate 303 is made smaller than the buffering plate bonding surface in the extent of 1 mm, is that if displacement is caused in the buffering plate 303 upon bonding, the retainer ring 304 cannot be engaged unless the buffering plate is formed smaller than the buffering plate bounding surface. It may be possible to bond the buffering plate 303 having greater external diameter than the internal diameter of the buffering plate bonding surface and subsequently cut the portion of the buffer plate extending from the bonding surface. However, in such case, cut edge tends to be irregular to make the polishing speed at the edge portion of the polishing object 101 non-uniform. Accordingly, in the shown construction of the fixing block 302, it becomes necessary to provide a dimensional difference in the extent of 1 mm between the buffering plate 303 and the buffering plate bonding surface. When the external diameter of the buffering plate 302 is smaller than the internal diameter of the buffering plate bonding surface of the fixing block in the extent of 1 mm, a gap portion of 1 mm at the maximum is formed. As a result, as shown by the pressure profile in the region of -75 to -71 mm of FIG. 6B, the pressure to be exerted on the polishing object 101 located above the gap portion, from the abrasive cloth, can be lowered not only in the region of 1 mm from the edge but also in the region of 1 to 4 mm from the edge due to deformation of the buffering plate 303 toward the gap portion 316. As a result, as shown in the region of -75 to -71 mm of FIG. 7, the polishing speed of the polishing object 101 in the region of 4 mm from the edge becomes lower than that of other portion and thus becomes non-uniform. Even in the method using Suba400 as the abrasive clot 108 in the lower layer, the polishing speed becomes low in the region of -75 to -71 mm due to displacement of the buffering plate 303. However, since the polishing speed in the region where the edge portion of the polishing object 101 is located above the buffering plate is also low, influence of displacement for non-uniformity of the polishing speed is lesser than that of the case where Suba800 is used as the abrasive cloth 108 of the lower layer.
Also, as a solution for non-uniformity of the polishing speed in the edge portion of the polishing object, there is a method to contact a wider width retainer ring to the abrasive cloth to exert a pressure, as proposed on a material of VMIC conference, Jun. 27 to 29, 1995, pp 525 to 527. This is the method to control the polishing speed by applying pressure on the base plate 415 and the retainer ring 404, as shown in FIG. 8A. The polishing method under typical polishing condition will be discussed with reference to FIG. 8A. On the abrasive cloth 107 rotated at 30 r.p.m., the polishing fluid is supplied from the polishing fluid supply nozzle (not shown) in a rate of 200 cc/min. Under this condition, the polishing object 101 held on the base plate 415 via the buffering plate 403 and the retainer ring 404 and the abrasive cloth 107 are fitted under pressure. By supplying air into an air bag 412 for the retainer ring and an air bag 413 for the base plate to apply a load of 7 psi on the retainer ring 404 and the base plate 415. The fixing block 402 is driven to rotate on the own axis. Under such condition, polishing is performed.
In this method, since local deformation of the abrasive cloth 107 and 108 at the edge portion of the polishing object 101 is received by the outer peripheral portion of the lower surface of the retainer ring 404, the pressure becomes uniform in the region of +60 to +75 mm. As a result, the polishing speed of the edge portion on the plus side becomes the same as in-plane region as shown by the profile.
However, since the buffering plate 403 is used even in such retainer ring contact method, similarly to the method using Suba800, displacement may be caused between the buffering plate bonding surface of the base plate and the upper surface of the polishing object 101 to form a gap portion 416 in the extent of 1 mm as shown in the enlarged section of the edge portion on the minus side in FIG. 8A. As a result, as shown by the pressure profile in FIG. 8B, the pressure on the edge and the area in the vicinity thereof is lowered due to presence of the gap portion 416. However, since the retainer ring 404 restricts deformation of the abrasive cloths 107 and 108, the deformation amount of the abrasive cloth 107 located over the edge portion is smaller than that in the case where Suba800 is used. Therefore, the region where the pressure is lowered is in a range of 3 mm from the edge, which is smaller than the range of 4 mm in the case where Suba800 is used. Accordingly, as shown in the region of -75 to -72 mm of FIG. 9, the polishing speed of the polishing object within the range of 3 mm from the edge becomes lower than that in other region and thus to be non-uniform.
In the construction of the fixing block in the conventional method, since the external diameter of the buffering plate 303 is smaller than the diameter of the buffering plate bonding portion of the fixing block 302 in the extent of 1 mm as shown in FIG. 6A maximum 1 mm of gap portion 316 is formed at certain bonding position of the buffering plate 303. By influence of this gap, in the method using Suba800 as the abrasive cloth 108, the polishing speed of the polishing object 101 in the region 4 mm from the edge becomes lower than that in other region to make polishing speed non-uniform. On the other hand, even in the method, in which the retainer ring 404 is contacted with the abrasive cloth 107 as shown in FIG. 8, the polishing speed of the polishing object 101 in the region 3 mm from the edge becomes lower than that in other region to make polishing speed non-uniform.
Accordingly, in the conventional polishing process, a region inappropriate for fabrication of the semiconductor device created in the polishing object is a region of 4 mm from the edge in the method using Suba800 and 3 mm from the edge in the method where the retainer ring 404 is contacted with the abrasive cloth. However, a currently available region where the semiconductor device can be fabricated in the fabrication process other than polishing process, is 2 mm from the edge. Therefore, the region of 2 to 4 mm from the edge lacks flatness to be a cause of defects in the product.